Self-tapping screw

ABSTRACT

A self-tapping screw having a boring part and a threaded shaft for fixing a panel-type object to a preferably metallic substructure is provided. The screw is configured in such a way that it produces a through hole in the panel type object, said hole accommodating the shaft with a degree of play and so that it produces a bore in the substructure into which the thread on the shaft cuts a counter-thread. The boring part includes a boring tip that is located eccentrically in relation to the longitudinal axis of the screw. The tip centers the screw during the positioning operation in such a way that the boring part executes a circular displacement about the boring tip, which expands the through hole to the desired size, whereas the bore in the substructure is produced with the diameter of the boring part. The screw replaces self-tapping screws provided with boring flights and is preferably for use with a substructure consisting of thin sheet metal through which the boring flights of conventional screws can inadvertently penetrate.

The invention relates to a self-tapping screw with a boring part and athreaded shaft for fixing a panel-type object to a substructure. Thescrew is configured so that it produces a through hole in the panel-typeobject that accommodates the threaded shaft with a degree of play. Thescrew is also configured so that it produces a bore hole in thesubstructure, in which the thread of the shaft cuts a counter-thread.

For the production of building paneling, typically panels are attachedby means of screws to a substructure rigidly connected to the building.The panels consist of, e.g., ceramic, asbestos cement, stone, or asimilar brittle material. The building paneling typically consists ofmetal, frequently of profiled sections made from thin sheet metal. Thebuilding paneling is exposed to large temperature fluctuations. When thesun is shining, the building paneling expands greatly relative to thesubstructure, which is covered by the building paneling, and thencontracts again as the sunshine diminishes. Thus, the building panelingwanders on the substructure. To reduce thermal stresses in the buildingpaneling, the attachment screws must not hinder the movements of thepaneling. For this purpose, through holes, which accommodate theattachment screws with a sufficient degree of play, are provided in thepanels. In the substructure itself, threaded bore holes are produced, inwhich the attachment screws are screwed.

For smaller building panels, the through holes can be produced in thepanels and the threaded holes can be produced in the substructure bydrilling or pre-drilling and then manually cutting the threads at theconstruction site. For larger building panels, self-tapping screws areused, which have a boring part and a shaft provided with a cuttingthread. The self-tapping screws are configured so that they themselvesproduce in the panel-type object a through hole, which accommodates thethreaded shaft with a degree of play, and so that they themselvesproduce in the substructure a bore hole, in which the thread of theshaft cuts a counter-thread. A self-tapping screw of the above-mentionedtype, like that known from EP 0 949 218 B1, is suitable for thispurpose. For this screw, freely projecting wings connect to a panel-typeboring part at an axial distance from the boring tip perpendicular tothe screw axis. The edge of the blades facing the boring tip is formedas a boring knife. At the connecting point of the wings to the boringpart, there is a predetermined breaking notch extending approximatelyparallel to the screw axis. The boring tip of the boring part firstdrills a hole in a cover track made from light metal to be attached to asubstructure. The hole is then expanded by the boring wings into athrough hole accommodating the screw with a degree of play. Then theboring part with the boring tip penetrates into the substructure,wherein, after the passage of the boring part, the wings contact thesubstructure and break off at the predetermined breaking notches. Forfurther rotation of the screw, the thread part of the shaft is thenguided into the bore hole produced in the substructure, so that a threadis cut into the substructure and the final attachment can be realized.For the use of such self-tapping screws for the production of buildingpaneling on a substructure made from thin sheet metal profiled sections,it has been shown that frequently the torque exerted on the wings by thethin sheet metal substructure is not enough to break off the wings.Consequently, the wings also penetrate and expand the bore hole in thesubstructure, so that the thread of the shaft cannot be held in thesubstructure.

The object of the invention is to create a self-tapping screw of thetype mentioned in the introduction such that a through holeaccommodating the shaft of the screw with a degree of play is producedonly in the panel-like object and not in a thin, preferably metallicsubstructure.

This object is achieved according to the invention by a self-tappingscrew of the type mentioned in the introduction, for which the boringpart has a boring tip arranged eccentrically relative to a longitudinalaxis of the screw.

The eccentric arrangement of the boring tip of the screw according tothe invention has the effect that the boring tip centers the screw in apanel-type object while this object is drilled through and the effectthat, during the drilling process, the longitudinal axis of the screwencloses an angle with the center axis of the bore hole, so that atleast one part of the boring part performs a circular motion about thecenter axis of the bore hole for a centered boring tip and thus producesan expanded through hole, whose diameter does vary in magnitude over thelongitudinal center axis of the bore hole, but can equal up to twice thediameter of the boring part according to the degree of eccentricity ofthe boring tip relative to the longitudinal axis of the screw. When theboring tip of the boring part penetrates the preferably metallicsubstructure and emerges increasingly at its bottom side, the centeringeffect of the boring tip increasingly lessens. The boring tip itselfthen makes the circular motion about the center axis of the bore hole,which, however, decreases with increasing drilling progress, because theangle enclosed between the longitudinal axis of the screw and the centeraxis of the bore hole decreases constantly. When both axes finallycoincide, the boring part still only performs a rotational motion aboutthe longitudinal axis of the screw and produces the bore hole in thesubstructure with a diameter, which is equal to the diameter of theboring part.

Advantageous configurations of the invention form the objects of thedependent claims.

In one configuration of the self-tapping screw according to theinvention, in which the boring tip is arranged at an external, radialedge of the boring part, the entire boring part performs the circularmotion mentioned above, so that the diameter of the bore hole in thesubstructure reaches twice the diameter of the boring part.

In another configuration of the self-tapping screw according to theinvention, in which the boring part is a thin boring plate, this alsohas the production advantages exhibited by the known screw described inthe introduction. Namely, the boring part can be manufactured togetherwith the wings in one production step as a mass-produced part.Therefore, no milling of the boring tip and no separate forming of thewings is required, which enables an economical and inexpensiveproduction of the entire self-tapping screw. In addition, it is possibleto provide such a thin boring plate for screws, which consist of lightmetal, non-ferrous metal, steel, stainless steel, or plastic.

In another configuration of the self-tapping screw according to theinvention, if the boring part is formed on the shaft, this configurationof the screw is suitable for production from a heat-treatable material,such as a carbon steel. Preferably, the boring part is produced bypressing the shaft and then heat treated. However, it can also beproduced on a separate shaft part and welded to the shaft of the screw,which has the thread or which receives the thread at a later time.

In another configuration of the self-tapping screw according to theinvention, if the boring part has a cutting edge, which has two axiallyoffset cutting regions, a small hole, which is then expanded to thediameter of the boring part with the axially offset second cuttingregion, can be pre-drilled in a simple way in the substructure with thecutting region adjacent to the boring tip.

In another configuration of the self-tapping screw according to theinvention, if the cutting regions are offset opposite each other in theaxial direction in a plane extending through a longitudinal axis of thescrew, the boring part can be produced especially simply as a sheet-typemass-produced part, e.g., through stamping.

In another configuration of the self-tapping screw according to theinvention, if the axial offset of the cutting regions is adapted to thethickness of the substructure to be bored, it is guaranteed that thebore hole in the substructure receives a diameter, which becomes equalat the most to the diameter of the boring part over the entire length ofthe bore hole, before the counter-thread is cut in the bore.

In another configuration of the self-tapping screw according to theinvention, if the boring part has a cutting edge, which is diagonalrelative to the longitudinal axis of the screw and which extends outfrom the boring tip over the entire width of the boring part, the sameresults can be achieved with this boring part as with the boring part inthe configuration of the invention, for which the cutting edge has twoaxially offset cutting regions.

In another configuration of the self-tapping screw according to theinvention, if an axial distance between the boring tip and a cuttingpoint of the cutting edge and the longitudinal axis of the screw isadapted to the thickness of the substructure to be bored through, it isguaranteed in turn that the bore hole in the substructure receives adiameter, which becomes equal at most to the diameter of the boring partover the entire length of the bore hole, before the counter-thread iscut in this bore hole.

Embodiments of the invention are described in more detail in thefollowing with reference to the drawings. Shown are

FIG. 1 is a view of a first embodiment of a self-tapping screw accordingto the invention,

FIGS. 2–9 show the process of attaching a panel-type object to asubstructure using the screw according to FIG. 1,

FIG. 10 shows a second embodiment of the self-tapping screw according tothe invention,

FIGS. 11–18 show the process of attaching a panel-type object to asubstructure using the screw according to FIG. 10, and

FIG. 19 shows a third embodiment of the self-tapping screw according tothe invention.

FIG. 1 shows a first embodiment of a self-tapping screw according to theinvention, which is designated overall with 20. The screw 20 has a head22, a shaft 26 provided with a thread 24, and a boring part 30. Theboring part 30 is a thin boring plate in the embodiment of FIG. 1. Thethin boring plate is fixed in a slot in the shaft 26 on the end of theshaft opposite the head 22. The boring part 30 has a cutting edge, whichis designated overall with 32 and which has two axially offset cuttingregions 34, 36. The screw 20 has a longitudinal axis 38. The cuttingregions 34, 36 are offset relative to each other in the axial directionin a plane extending through the longitudinal axis 38 of the screw 20.Here, the cutting regions 34, 36 offset relative to each other in theaxial direction are parallel to each other. According to theillustration in FIGS. 2–9, the screw 20 is used for fixing a panel-typeobject 40 to a substructure 42, which here consists of thin sheet metal(with a thickness of, e.g., 0.8–1.2 mm). The panel-type object 40 is apanel of building paneling, for example. The axial offset of the cuttingregions 34, 36 is adapted to the thickness of the substructure 42 to bebored. In the illustration in FIGS. 2–9, the axial offset isapproximately equal to the thickness of the substructure 42.

The previously described configuration of the screw 20 is used for thepurpose of producing in the panel-type object 40 a through hole 44 (FIG.8), which accommodates the shaft 26 with the thread 24 with a degree ofplay, and for producing in the substructure 42 a bore hole 46, in whichthe thread 24 of the shaft 26 cuts a counter-thread. For this purpose,the boring part 30 has a boring tip 50 arranged eccentrically relativeto the longitudinal axis 38 of the screw 20. In the embodiment shown inFIGS. 1–9, the boring tip 50 is arranged on an outer edge 52 of theboring part 30 in the radial direction. However, it is possible toarrange the boring tip 50 in a region between the outer edge 52 and thelongitudinal axis 38 of the screw instead. In this case, the throughhole 44 would turn out smaller in diameter. This will be made clear fromthe following discussion.

The effect of the eccentrically arranged boring tip 50 during thesetting of the self-tapping screw 20 is described in detail in thefollowing with reference to FIGS. 2–9.

Starting at the beginning of the boring process in the panel-type object40, FIG. 2, the boring tip 50 forms a centering tip for the screw 20.With the front cutting region 34 in the axial direction, which performsa circular motion about a vertical center axis 48 (cf. FIG. 5) of thelater through hole 44 extending through the boring tip 50, a small hole41 is pre-drilled. The longitudinal axis 38 of the screw 20 here formsan angle α with the center axis 48, as shown in FIG. 5. To set the screw20, any arbitrary turning tool can be used, which has sufficient play inthe receptacle part for the head 22 to allow the diagonal arrangement ofthe screw 20, as shown in FIGS. 2–8. The turning tool merely has to berotated about its own axis. It should not perform any kind of back andforth or circular motion. It is sufficient when the boring part 30performs a circular motion about the center axis 48. With increasingdrilling progress, finally the entire cutting edge 32 of the boring part30 is used to produce the through hole 44 in the panel-type object 40,as shown in FIGS. 3 and 4. Due to the circular motion of the boring part30, the through hole 44 is not cylindrical, but instead conical, whereinit increases in diameter in the direction towards the substructure 42.Then the boring tip 50 penetrates into the substructure 42, so that thesame boring process as in the panel-type object 40, whose beginning isshown in FIG. 2, repeats in the substructure 42, FIGS. 5 and 6. In FIG.6, the boring tip 50 has reached the bottom side of the substructure 42.The through hole 44 is now expanded by the circular motion of the boringpart 50 to its maximum size. In the shown embodiment, in which theboring part 50 is arranged at the outer edge 52 of the boring part 30 inthe radial direction, the largest diameter of the through hole 44 isequal to twice the diameter of the boring part 30. When the boring tip50 of the boring part 30 penetrates the substructure 42 and emerges moreand more at its bottom side, FIGS. 7 and 8, the centering effect of theboring tip 50 becomes less and less. The boring tip 50 itself then makesthe circular motion about the center axis 48 of the through hole 44,which decreases, however, with increasing drilling progress, because theangle α enclosed between the longitudinal axis 38 of the screw 20 andthe center axis 48 of the through hole 44 constantly decreases until theentire boring part 30 is finally accommodated in the substructure 42,FIG. 8. In this phase of the boring progress, the two axes 38 and 48coincide. The boring part 30 still only performs a rotational motionabout the longitudinal axis 38 and produces the bore hole 46 in thesubstructure 42 with a diameter, which is equal to the diameter of theboring part 30, FIG. 8. Finally, with further advance of the screw 20,the thread 24 of the shaft 26 penetrates into the bore hole 46 to cut acounter-thread in the bore hole. This thread finally sets the screw 20,FIG. 9, wherein it can be seen that the shaft 26 of the screw 20provided with the thread 24 has an over-dimensioned amount of play inthe through hole 44. This amount could be reduced if the boring tip 50were not arranged at the outer edge 52 of the boring part 30 in theradial direction, but instead more in the direction towards thelongitudinal axis 38 of the screw 20, as described above.

During the setting of the screw 20, starting with the placement of theboring tip 50 on the panel-type object 40, FIG. 2, the screw is arrangeddiagonally such that a vertical axis, e.g., the center axis 48 of thethrough hole 44 and the bore hole 46, is passed through on one side bythe boring tip 50 and on the other side by the center of the top side ofthe head 22 of the screw 20, as shown in FIG. 5. This guarantees thatthe head 22 merely turns about the center axis 48, thus does not performa back and forth or circular motion, in contrast the boring part 30performs the intended circular motion about the center axis 48 in orderto widen the through hole 44 finally, as is the case starting with theillustration in FIG. 7. If the axial offset between the cutting regions34 and 36 was selected smaller than the thickness of the substructure42, the pre-drilled hole 51 in the substructure would be expanded to thehole 46 with a diameter, which would be greater than the diameter of theboring part 30.

FIG. 10 shows a second embodiment of the self-tapping screw according tothe invention, which is here designated overall with 20′. The screw 20′is different from the screw 20 in that it has a boring part 30′, whichhas a cutting edge 33, which is diagonal relative to the longitudinalaxis 38 of the screw 20′ and which extends from the boring tip 50 overthe entire width of the boring part. The boring part 30′ is either athin boring plate fixed to the shaft 26 or is formed on the shaft 26.With the screw 20′ with the boring part 30′, the same results can beachieved as with the screw 20 with the boring part 30. So that this canbe achieved, an axial distance A between the boring tip 50 and a cuttingpoint 35 of the cutting edge 33 and the longitudinal axis 38 of thescrew 20′ is adapted to the thickness of the substructure 42 to bebored. Advantageously, the axial distance A is selected according to thesame points of view as the axial offset of the cutting regions 34 and 36of the boring part 30, so that the comments expressed there also applyhere.

The process of setting the screw 20′ is shown in FIGS. 11–18. Therepresentations in FIGS. 11–18 correspond to the representations inFIGS. 2–9, so that the comments expressed for the latter also apply hereand do not need to be repeated.

FIG. 19 shows a third embodiment of a self-tapping screw 20″ accordingto the invention. The screw 20″ has a boring part 30″ pressed on thethreaded shaft 26. The effect of the boring part 30″ of the screw 20″corresponds to the effect of the boring part 30 or 30′ of the screws 20or 20′, therefore the comments expressed there also apply for theembodiment according to FIG. 19.

1. Self-tapping screw comprising a head, a boring part and a threadedshaft for attaching a panel to a substructure, wherein the screw isconfigured such that it produces a through hole in the panel, whichaccommodates the threaded shaft with a degree of play, and produces inthe substructure a bore hole, in which the thread of the shaft cuts acounter-thread, wherein the boring part (30, 30′, 30″) has a boring tip(50, 50′, 50″) arranged at an outer edge at one radial side of the shaftso that it is eccentric relative to a longitudinal axis (38) of thescrew (20, 20′, 20″) to define an initial drive axis offset by an angleα to the longitudinal axis, and a cutting edge lying in a longitudinalplane extending from the boring tip and inclined across the screw to anopposite radial side of the shaft from the boring tip so that the boringpart is adapted to cut the through hole up to twice the diameter of theboring part.
 2. Self-tapping screw according to claim 1, wherein theboring part (30) is a thin boring plate.
 3. Self-tapping screw accordingto claim 1, wherein the boring part (30′, 30″) is formed on the shaft(26).
 4. Self-tapping screw according to claim 1 wherein the cuttingedge (32) has two axially offset cutting regions (34, 36). 5.Self-tapping screw according to claim 4, wherein the cutting regions(34, 36) are offset relative to each other in an axial direction in aplane extending through the longitudinal axis (38) of the screw (20). 6.Self-tapping screw according to claim 4, wherein an axial offset of thecutting regions (34, 36) is adapted to a thickness of the substructure(42) to be bored.
 7. Self-tapping screw according to claim 1, whereinthe cutting edge (33) is linear (28) of the screw (20′) and extends fromthe boring tip (50′) over an entire width of the boring part (30′). 8.Self-tapping screw according to claim 7, wherein an axial distance (A)between the boring tip (50′) and a cutting point (35) of the cuttingedge (33) and the longitudinal axis (38) of the screw (20′) is adaptedto a thickness of the substructure (42) to be bored.
 9. Self-tappingscrew comprising a head, a boring part and a threaded shaft forattaching a panel to a substructure, the screw is configured such thatit produces a through hole in the panel, which accommodates the threadedshaft with a degree of play, and produces in the substructure a borehole, in which the thread of the shaft cuts a counter-thread, the boringpart (30, 30′, 30″) has a boring tip (50, 50′, 50″) arranged at an outeredge at one radial side of the shaft so that it is eccentric relative toa longitudinal axis (38) of the screw (20, 20′, 20″), and a cutting edgelying in a longitudinal plane extending from the boring tip and inclinedacross the screw to an opposite radial side of the shaft from the boringtip, the boring tip forming an initial rotation axis to cut the throughhole.